The reason you seem to get the effect of looping is down to harmonic content of string sounds and also if you take the string sound on pro recordings where there are only 7 or 8 string players you can get a looping effect due to the fact that string players have to change the direction of the bow,which means the tone structure of the instrument changes of slightly and in an orchestra you don't notice it so much as not only does each musician change bow direction at slightly times but as you have multi parts all the harmonics resonate together. Brass sounds are not produce sound using a sound board and as such do not resonate in sympathy as stringed instruments with other member of the same section.
Also if you think string section sounds are often played on a keyboard in an unreal way because orchestral strings also have parts which include more than just strings,there is woodwind horns, again making up the richness of the sound